It is well known that the asynchronous operation of bladed propulsors, such as propellers and fans, on multi-engine aircraft generates acoustic cabin noise and cabin vibrations which may be annoying to passengers. Each of the propellers or fans creates airflow disturbances and beats as its blades rotate through the air flowing past the propeller or fan. Also, rotor imbalance acts on the propulsor shaft and is transmitted to and excites the aircraft structure. As a result, acoustic noise and vibrations are generated which are felt in the aircraft cabin.
The conventional method of minimizing cabin noise generated by the asynchronous operation of the bladed propulsors on multi-engine aircraft is to maintain a phase angle difference between the bladed propulsors which results in an interaction of the airflow disturbances created by the propulsors so as to at least partially cancel each other thereby reducing to varying degree the noise transmitted to the cabin. Similarly, by proper phase angle selection, interaction may be generated which results in mutual cancellation of the mechanical excitations from the imbalance in the propulsors so as to reduce cabin vibration. Typically, one propulsor is designated as the master propulsor, and the phase angle relationship of the blades of other propulsor or propulsors, termed slave propulsor or propulsors, as the case may be, is fixed relative to the blades of the master propulsor to minimize cabin noise. That is, the blades of each slave propulsor are circumferentially offset from the corresponding blades of the master propulsor by a desired phase angle which has been determined to be that phase angle offset at which the beating noise characteristic of asynchronous operation is minimized.
Unfortunately, minimum cabin noise production and minimum cabin vibration generation are not often experienced at the same phase angle offsets between master and slave propulsors. The production of noise is a function of the frequency of blade passage through the air. Thus, the dominant noise pattern is a harmonic which repeats an integral number of times directly proportional to the number of blades of the propulsor. For a four blade propulsor, the minimum phase angle offset at which minimum noise will be experienced lies between 0 and 90 degrees. However, as vibration generation is a harmonic of the rotational speed of the propulsor, the phase angle offset which produces minimum vibration may lie at any value between 0 and 360 degrees. Therefore, setting the phase angle offset between the blades of the slave propulsor and the blades of the master propulsor at the minimum value for the phase angle offset which produces minimum noise seldom results in the simultaneous reduction of cabin vibration to low levels and may even result in a setting which produces near maximum vibration.
Accordingly, it is an object of the present invention to provide a method for phase angle synchronizing the rotation of a N-bladed slave propeller with the rotation of a N-bladed master propeller of an aircraft so as to reduce both noise and vibration in the passenger cabin of the aircraft.
It is also an object of the present invention to provide such a method wherein the phase angle at blades of the slave propeller from the blades of the master propeller is selected to be that phase angle at which cabin noise generation is at a minimum and cabin vibration is reduced to near minimum levels.